Overview. Overview of Transaction Management. Definition of a transaction. What is a transaction? Chapter 16

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1 1 2 Overview Overview of Transaction Management Chapter 16 Transactions and the atomicity concept The so-called ACID principle Backup and recovery mechanisms» the types of failures that may occur» describe and evaluate the types of backup, recovery and restart procedures Concurrency control- allowing concurrent updates» Lost updates» Inconsistent retrievals» Uncommitted data» Deadlock What is a transaction? 3 Definition of a transaction 4 A transaction corresponds to a real world event Initial state Balance = Transaction: Balance = Balance End state: Balance = ??? = ABM Steps for correct transaction Example: Withdraw $ from your savings account at an ABM ABM closes during transaction before you have received you money A transaction is a logical unit of work Either the whole transaction is executed, or no part thereof should be reflected in the database balance $ balance $ successful transaction withdraw $ unsuccessful transaction no withdraw $ balance $ balance $

2 5 Transaction properties to ensure consistency What Is a Transaction? 6 Atomic Consistent ACID-principle Durable Isolated Transaction Properties» Atomicity requires that all operations of a transaction be completed; if not, the transaction is aborted.» Durability indicates the permanence of the database s consistent state.» Serializability describes the result of the concurrent execution of several transactions. This property is important in multi user and distributed databases.» Isolation means that the data used during the execution of a transaction cannot be used by a second transaction until the first one is completed. 7 8 What Is a Transaction? What Is a Transaction? Transaction Management with SQL» Transaction support is provided by two SQL statements: COMMIT and ROLLBACK.» When a transaction sequence is initiated, it must continue through all succeeding SQL statements until one of the following four events occurs: A COMMIT statement is reached. A ROLLBACK statement is reached. The end of a program is successfully reached (COMMIT). The program is abnormally terminated (ROLLBACK). Transaction Management with SQL» Example: UPDATE PRODUCT SET PROD_QOH = PROD_QOH WHERE PROD_CODE = 345TYX ; UPDATE ACCREC SET AR_BALANCE = AR_BALANCE WHERE AR_NUM = ; COMMIT;

3 9 Transactions in SQL: Withdraw $ from account 6676 The Transaction log 10 Begin Transaction Update Client Set Current-Balance = Current-Balance - $ Where Account = 6676 and Current-Balance > $400.00; COMMIT; A transaction log keeps track of all transactions that update the database. The information stored in the log is used by the DBMS for a recovery requirement triggered by a ROLLBACK statement or a system failure. The transaction log stores before-and-after data about the database and any of the tables, rows, and attribute values that participated in the transaction. The transaction log is itself a database, and it is managed by the DBMS like any other database. The Transaction log 11 Transaction log: withdraw $ from account Transaction log is used to record actions during execution Begin Transaction Update Client Set Current-Balance = Current-Balance - $ Where Account = 6676 and Current-Balance > $400.00; COMMIT; Transaction log TID Table TupleID Attribute Before T After T Table 9.1 Transaction Log 101 **** begin transact 101 Client 32134Y Current-Balance **** end transact

4 13 Database Recovery Management 14 So what happens if some one turns the power switch off? Recovery restores a database from a given state, usually inconsistent, to a previously consistent state. Recovery techniques are based on the atomic transaction property: All portions of the transaction must be applied and completed to produce a consistent database. If, for some reason, any transaction operation cannot be completed, the transaction must be aborted, and any changes to the database must be rolled back. Database recovery 15 Database Recovery Management 16 Principle: Prevention is better than cure Errors Transaction errors System errors Operator errors Program errors Power failure Hardware error Network error Disaster Solution: BACKUPS Failures Transaction terminated; no loss of data System stopped; data in main memory lost System stopped; data on disk lost Severe loss of data (Fire, earthquake) Database Failures» Software Operating system, DBMS, application programs, viruses» Hardware Memory chip errors, disk crashes, bad disk sectors, disk full errors» Programming Exemption Application programs, end users» Transaction Deadlocks» External Fire, earthquake, flood

5 17 18 Recovery prevention: Types of backup Database Recovery Management Full Backup Full Database Incremental backup Changes since last backup Transaction log transaction log Recovery Procedures:» Deferred write and Deferred update Transaction operations do not immediately update the database. Instead, all changes are written to the transaction log. The database updated only after the transaction reaches its commit point.» Write through The database is immediately updated by transaction operations during the transaction s execution, even before the transaction reaches its commit point. The transaction log is also updated. If a transaction fails, the database uses the log information to roll back the database. How do we recover from a failure? Steps: 1: Determine the cause of the failure. 2: Determine the damage caused by the failure 3: Start from a previous consistent database state (Usually a previous backup) 4: If a transaction log is available, then execute the transactions that are contained in the log. So what if two transactions want to update the same data?... Are your data secure?????

6 21 22 Overview of Concurrency control Concurrency Control Explain the need for concurrency control Explain what the lost updates, uncommitted data and inconsistent retrievals problems entail Explain the operation of the transaction scheduler Describe, evaluate and contrast the following concurrency control techniques» locking» timestamps» optimistic validation Concurrency control coordinates simultaneous execution of transactions in a multiprocessing database.» The objective of concurrency control is to ensure the serializability of transactions in a multi user database environment.» Simultaneous execution of transactions over a shared database can create several data integrity and consistency problems: Lost Updates. Uncommitted Data Inconsistent retrievals Deadlock Transaction management problem 1: Lost updates Transaction management problem 2: uncommitted data Initial balance $ Initial balance $ Transaction 1: Transaction 2: Transaction 1: Transaction 2: Withdraw $ from account 6676 Correct execution of transaction Deposit $ into account 6676 Lost update Start to withdraw $ from account 6676; but decide against it and cancel transaction. Correct execution of transaction Deposit $ in account 6676 Uncommitted data Time TID Step Value stored Time TID Step Value stored Time Tid Step Value stored Time Tid Stap Value stored 1 T1 Read Balance T1 Balance T1 Write Balance T2 Read Balance T2 Balance T2 Write Balance T1 Read Balance T2 Read Balance T1 Balance T2 Balance T1 Write Balance T2 Write Balance T1 Read Balance T1 Balance T1 Write Balance T1 **ROLLBACK** T2 Read Balance T2 Balance T2 Write Balance T1 Read Balance T1 Balance T1 Write Balance T2 Read Balance T2 Balance T1 ***ROLLBACK*** T2 Write Balance Lost update Read uncommitted data

7 25 26 Transaction management problem 3: Inconsistent retrievals Transaction management problem 3: Inconsistent retrievals Woolphits General Dealers Transaction1: Determine the inventory at the Ottawa branch Transaction 2: Update the stock at Ottawa branch: number of towels glass bowls (a a staff member dropped a container) T1: Select SUM(Quantity-on on-hand) From Inventory; Commit; Requirement results: Product Before T2 After T2 T2: Update Inventory Set Quantity-on on-hand = Quantity-on on-hand Where Product = Towels ; Update Inventory Set Quantity-on on-hand = Quantity-on on-hand Where Product = Glass-bowls ; Commit; Cutlery Towels Glass bowls Duvets TOTAL Transaction management problem 3: Inconsistent retrievals 27 The solution: Use a transaction scheduler 28 T1: Select SUM(Quantity-on on-hand) T2: Update Inventory From Inventory; Set Quantity-on on-hand = COMMIT; Quantity-on on-hand Where Product = Towels ; Update Inventory Set Quantity-on on-hand = Quantity-on on-hand Inconsistent retrievals: Where Product = Glass-bowls ; COMMIT; Time TID Action Value Total 1 T1 Read Cutlery T2 Read Towels T1 Read Towels T2 Towels = T1 Read glass bowls T2 Read glass bowls T2 Glass bowls = T2 ***COMMIT*** 9 T1 Read duvets T1 ***COMMIT** Determine order of concurrent execution T1 T2 Conflict Read Read no Read Write yes Write Read yes Write Write yes Scheduler ensures serializability*: The result of concurrent execution is equivalent to a serial execution That is, it appears as if the transactions are serially executed *different types of serializabilitynext time

8 29 30 The Scheduler Scheduling method 1: Concurrency control using locking The scheduler establishes the order in which the operations within concurrent transactions are executed. The scheduler interleaves the execution of database operations to ensure serializability. To determine the appropriate order, the scheduler bases its actions on concurrency control algorithms, such as locking or time stamping methods. The scheduler also makes sure that the computer s CPU is used efficiently. (Design: Different DBMS, different speeds ) Time Transaction 1 Transaction 2 1 Request lock for table A 2 OK Request lock for table A 3 Lock table A Wait 4 5 Execution of transaction 6 Release lock OK 7 OK 8 Lock table A 9 10 Database 11 Execution of transaction 12 Table A Release lock Tabel B Scheduling method 1: Concurrency control using locking Locking problem 1: Serializability cannot be guaranteed Solution: Two phase locking Level of locking database table page row attribute Level of locking determines availability. Why? Growing phase Locked phase Shrinking phase Lock point Types of locking: Binary: Shared: Exclusive: locked versus available allow reads, no writes no reads, no writes Require locks Unlock

9 33 Two Phase Locking Rules for Two Phase Locking Protocol 34 The two phase locking protocol defines how transactions acquire and relinquish locks. It guarantees serializability, but it does not prevent deadlocks. In a growing phase, a transaction acquires all the required locks without unlocking any data. Once all locks have been acquired, the transaction is in its locked point. In a shrinking phase, a transaction releases all locks and cannot obtain any new locks. Two transactions cannot have conflicting locks. No unlock operation can precede a lock operation in the same transaction. No data are affected until all locks are obtained that is, until the transaction is in its locked point. Locking problem 2: Deadlock 35 Three Techniques to Control Deadlocks 36 Deadlock techniques Deadlock prevention Deadlock detection Deadlock avoidance Transaction 1 Transaction 2 has A has B wait for B wait for A Deadlock Prevention A transaction requesting a new lock is aborted if there is a possibility that a deadlock can occur. Deadlock Detection The DBMS periodically tests the database for deadlocks. If a deadlock is found, one of the transactions ( victim ) is aborted, and the other transaction continues. Deadlock Avoidance The transaction must obtain all the locks it needs before it can be executed.

10 37 Scheduling methods 2,3: Timestamps and optimistic methods 38 Global unique timestamps Optimistic methods T2 Timestamp = 12 T1 IF T1 and T2 are in conflict, then abort one of the transactions and reschedule it. Timestamp = 10 T1 T1 executes until commit Steps Read: execute transaction Validate: consistent? Write: commit valid transactions Next Theoretical foundations More in depth analysis

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